The invention relates to a high-pressure discharge lamp comprising a discharge vessel provided with a first and a second lamp electrode, an outer envelope surrounding the discharge vessel with an interspace and having a lamp cap, and a tubular UV enhancer located in the interspace and having a longitudinal axis, said UV enhancer having a wall of ceramic material and being provided with an internal enhancer electrode connected to the first lamp electrode and a capacitive coupling connected to the second lamp electrode, the enhancer electrode having an extremity which is situated within the UV enhancer.
A known problem in high-pressure discharge lamps in general is the ignition of these lamps. Dependent on the type of lamp, a relatively high ignition voltage is required, which is generally supplied in the form of one or more ignition voltage pulses to the lamp by a starter. In practice, there may be an inadmissibly long ignition time, even when the ignition voltage pulses are sufficiently high, while furthermore a large spread of this ignition delay is obtained. This is the result of a shortage of primary electrons in the discharge vessel, introducing the lamp discharge during ignition. By adding a small quantity of 85Kr in the discharge vessel, the shortage of primary electrons can be eliminated so that the ignition time will become shorter and its spread is reduced. 85Kr has the drawback that it is radioactive, and its use can be avoided by using an UV enhancer. This is a relatively small discharge vessel that produces UV radiation and is placed in the proximity of the discharge vessel of the lamp. When the lamp is ignited, the UV radiation emitted by the UV enhancer ensures that there are sufficient primary electrons in the discharge vessel of the lamp.
A lamp of the type described in the opening paragraph is known from WO 98/02902. The known lamp is a high-pressure discharge lamp, more particularly a metal halide lamp. This lamp has a discharge vessel with two lamp electrodes. The material of the discharge vessel may be quartz glass or a ceramic material. In this description and the claims, a ceramic material is understood to mean a densely sintered polycrystalline metal oxide, such as aluminum oxide or yttrium aluminum garnet, or a densely sintered polycrystalline metal nitride such as aluminum nitride. An outer envelope supporting a lamp cap surrounds the discharge vessel. The space between the discharge vessel and the outer envelope accommodates an UV enhancer, which has a wall of ceramic material and is provided with an enhancer electrode, which is connected to a first lamp electrode, and with a capacitive coupling. This capacitive coupling is realized by placing the UV enhancer in the proximity of a supply wire to a second lamp electrode. The use of a capacitively coupled UV enhancer as compared with an enhancer with two internal electrodes has the advantage that the enhancer is only operative when this is necessary, namely when ignition voltage pulses having a relatively high voltage and a high frequency are presented. Consequently, the enhancer does not consume energy during operation of the lamp and thus has a very long lifetime.
The use of a ceramic material for the wall of the UV enhancer has a favorable influence on the ignition behavior of the lamp, because the UV radiation generated by a ceramic UV enhancer appears to considerably increase the possibility of introducing the lamp discharge (lamp breakdown). However, the known lamp has the drawback that the UV enhancer itself may have an unacceptably high ignition delay, particularly when the lamp with the UV enhancer has been in the dark for some time. This leads to an unacceptable ignition delay of the lamp and to a large spread of the ignition time of the lamp.
It is an object of the invention to provide measures of eliminating the above-mentioned drawback. According to the invention, a high-pressure discharge lamp of the type described in the opening paragraph is characterized in that the capacitive coupling is constituted by a metal curl surrounding the UV enhancer in a plane transverse to the longitudinal axis of the UV enhancer and being situated in the proximity of the extremity of the enhancer electrode within the UV enhancer.
It has been found that a very satisfactory capacitive coupling of the UV enhancer is obtained by placing a metal curl around the wall of the enhancer and by connecting this curl to the second lamp electrode. The metal curl must be situated in the proximity of the extremity of the enhancer electrode within the UV enhancer. Due to the eminent capacitive coupling of the UV enhancer in a lamp according to the invention, the possibility of breakdown of the lamp when using ignition pulses considerably increases, while furthermore the height of the ignition pulses, which is minimally required for a reliable ignition, may be relatively small.
Lamps according to the invention are preferred, in which the distance between the extremity of the enhancer electrode within the UV enhancer and the plane in which the metal curl is situated is at most equal to the external diameter of the UV enhancer. In such lamps, the capacitive coupling of the UV enhancer is in fact optimal, particularly if said extremity of the enhancer electrode is situated in the plane of the metal curl, so that the metal curl is minimally spaced apart from the electrode extremity.
In a preferred embodiment, the UV enhancer has a wall of densely sintered polycrystalline aluminum oxide. This material is often used in the manufacture of high-pressure discharge lamps, so that an existing technology for ceramic discharge vessels can be employed, allowing miniaturization within strict tolerance limits.
A very advantageous embodiment of a lamp according to the invention, in which the enhancer electrode has a lead-through at a first extremity of the UV enhancer, is characterized in that the extremity of the enhancer electrode within the UV enhancer is spaced apart from the first extremity of the UV enhancer by a distance which is at least equal to twice the external diameter of the UV enhancer. In such a construction, the possibility of an unwanted breakdown between the metal curl and the lead-through to the enhancer electrode is very small when ignition pulses are supplied.
A combination of mercury and a rare gas is possible as a filling for the UV enhancer. However, a rare gas or a mixture of rare gases is preferred, because this precludes the use of the heavy metal mercury. Very satisfactory results are obtained when using argon as a filling for the UV enhancer. The filling pressure of the rare gas filling is then preferably chosen to be in the range from 50 to 300 mbar. At pressure values of less than 50 mbar, the UV output of the enhancer appears to become smaller; at pressure values of more than 300 mbar, the ignition voltage of the enhancer may assume too high values.
In a further preferred embodiment of a lamp according to the invention, the UV enhancer is situated in the proximity of a lamp electrode, with its longitudinal axis being substantially parallel to the longitudinal axis of the lamp. In this embodiment, it is achieved that a maximal quantity of the UV radiation generated in the enhancer directly impinges upon the lamp electrode, which is favorable for generating secondary electrons in the lamp.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.